Conversion

Basics

Priests are very expensive units (cost: 125 gold), thus you want to use them well and not lose them. However, they are so slow that they cannot run away, and they cannot find and have very few hit points (25 at the beginning, 50 after researching Mysticism).

A good use of Priests is healing. Your Priest can heal without pausing, but he does not heal fast enough to win a hopeless battle. However, it makes a difference when the forces are equal. The main advantage of healing is that you will not attract the attention of the enemy. Note that you can also heal catapults even though they are not organic, but not ships.

Conversion is much more powerful than healing, though. You do not only eliminate an enemy but gain another unit as well. As soon as your Priest attempts to convert an enemy, the enemy will either attack instantly or flee (if it is a transport). So you have to make sure that your Priest is protected by walls or Infantry units such as Hoplites. After converting an enemy, the Priest's faith will have to replenish before he can start a second attempt. Usually you can only convert one or two units in a battle.

The time that is needed for converting an enemy is not constant. Sometimes it takes longer than usual, so you can never be quite sure that a conversion will be successful in time. However, failed conversions do not drain the Priest's faith.

Which units should you convert?

Obviously converting a villager is a waste, and trying to convert a Cavalry unit is hopeless since they are so fast that they will easily kill the Priest. You have to concentrate on slow but powerful units such as Hoplites and War Elephants. War Ships are also good victims because they have a shorter range. However, they are twice as resistant to conversion as other units, so you should use several Priests to convert a ship.

If you see a Catapult that is guarded by a Phalanx, go for the Phalanx! The Phalanx will destroy the Catapult and be a valuable addition for your army.

Transports

Converting a transport is certainly promising since you can control the fate of up to ten enemy units if you are successful.

Whenever one use a Priest to capture a enemy transport, the units within the ship are not converted. So it would be a good idea to disembark far from your city or make sure you can kill or convert the units
being onboard the transport vessel.

Torkild

Since a transport is pretty cheap, there are other things that you can do then:

sink the transport with "delete"

let the enemy sink the transport (if he is stupid enough) and kill his own men :)

in FFA games, use the enemy's troops against another enemy. Let them eliminate each other ;)

hide the transport! This is very mean since the unit limit for the enemy
will thus be reduced by 5 or 10.

I think that the last idea is the best one. Having the unit limit reduced without even knowing why is certainly annoying ;)

If someone tries to capture your transport, flee at once or unload it! You will not want to lose the control of all your guys.

Towers

Monotheism lets you convert buildings and enemy priests. However, buildings cannot be converted at a distance, so take care of your priests! Some tricks to have your priests survive are:

Get Astrology to increase the effectiveness of conversion, Mysticism to double their hit points and Polytheism to make your priests walk faster.

Make sure that you can defend your priests against attacking Chariots and Cavalry.

Have at least two, better three priests ready (depending also on the upgrades that the tower already has).

Put your priests next to each other and send one to convert the tower and the other ones to heal him. They can convert other units afterwards.

If your opponent does not pay attention, you can draw the fire of the tower on a different unit (send it into the range of the tower before sending in the priests).

Do not try to convert towers standing next to each other because the priests will not survive it. Use Catapults with their radius damage instead.

Some of these tricks also apply to converting archers.
The main advantage when converting a tower is if your opponent relied on that tower for his defense. Maybe you can even attack a few villagers with your towers now, or it was the last tower that was still standing. It will help your attack very much.

Breaking the 50 units limit

This trick for team games is from E Pluribus:

Both you and your ally have maxed out the units. What to do?

Have your ally send a few priests somewhere to meet a few of your worker bees. You construct a wall around him to
trap him. Then your workers construct an infantry or cavalry building nearby.

Now, you make sure you and your ally do not have forces near each other, then switch to ENEMY. OK this sounds
crazy... hold on I'll explain.

Now send some infantry to the walled-in priest and have your "enemy" ally convert them to his cause! Your guys cannot
attack the priests because they are trapped in YOUR walls, so they are easily converted. You can even try making little
stalls out of walls for them so they do not attack each other when some are converted.

Oh cool... he has more than 50 units!
Oh wow... you have less than 50 and can generate more!

E Pluribus

You can also use this trick to get units that your tribe cannot train, e. g. Heavy Transports.

Special tricks

Priest tactic - Group your priests in pairs and have two priests convert
the same unit simultaneously. Often, the target of the conversion cannot
make up his mind which priest to attack and just stands there confused.
Easy pickings for conversion.

Anti-priest tactic - For this tactic, you need a fast-moving high-damage
unit (e.g., Cavalry), a slow-moving low-damage non-range
easily-converted unit (e.g., swordsman), and a few priests of your own.
Send the slow unit towards the enemy priests. All of the enemy priests
will try to convert your slow unit. Let it be converted. Once it gets
converted, have your priests try to convert it back. Meanwhile, send
your fast-moving high-damage unit to attack the enemy priests. The
enemy priests are helpess against your fast unit since they are still
recovering from the previous conversion. This tactic worked great in
the "I'll be back" scenario.